To explore the primary stage or site of action of acetamizuril (AZL), a novel triazine anticoccidial compound, the ultrastructural development of Eimeria tenella at different endogenous stages was studied in experimentally infected chickens treated with a single oral dose of 15 mg/kg AZL. As a result of drug action, the differentiations of second-generation schizonts and microgamonts were largely inhibited and merozoites became irregular in shape. Meanwhile, the outer membrane blistering and perinuclear space enlargement were obvious in the second-generation schizonts and microgamonts, which were never observed in the classic triazine anticoccidiosis drug diclazuril-treated E. tenella. The chromatin aggregation, anachromasis, and marginalization were visible in merozoites and microgamonts. Furthermore, the abnormal evagination of microgametes finally resulted in the degeneration of microgamonts and the failure of subsequent fertilization. The most marked micromorphological alteration occurring in the macrogamonts was the WFB2. Even if the fertilization occurred, the formation of oocyst wall became malformed and the zygote proceeded to the obvious degeneration. In addition, mitochondria swelling and cytoplasm vacuolization were discerned in respective intracellular stages, while endoplasmic reticulum and Golgi body swelling was less seen. These alterations may be the causes leading to the final death of E. tenella and also provide some information for further exploring the mechanism of action of AZL at the molecular level.
Keywords: Acetamizuril; E. tenella; Endogenous phase; Ultrastructure.